Method of making washed shell mold



United States Patent Ofltice 2,847,741 Patented Aug. 19, 1958 2,847,741METHOD OF MAKING WASHED SHELL MOLD Application November 29, 1954 SerialN 0. 471,930

4 Claims. (Cl. 22-193) No Drawing.

This invention relates to shell molding and more particularly to theelimination of characteristic surface defects in low carbon, low alloysteel castings produced in conventional shell molds.

As is known in the art, such molds are generally formed of a mixture ofsilica or zirconite sand and a thermo-setting phenolic resin, either inpowder form or in a solvent to achieve uniform coating of the grains ofsand.

In some mixtures dust suppressants are used for health purposes andsometimes a release agent is incorporated in the mixture to prevent theshell from sticking to the pattern which is heated during forming of theshell mold to set the resin.

Although such shell molds ordinarily produce commercially acceptableiron castings and stainless steel castings, it is Well known in the artthat low carbon, low alloy steel castings, such as those formed, forexample of grade 8" steel, are subject to characteristic surface defectswhen formed in such molds. It is also known that this and otherdifficulties exist with respect to certain grades of stainless steel andmalleable iron. Prior art efforts to solve this problem have largelybeen directed in one of two channels. One approach to the problem hasbeen to discover a substitute for the sand, such as, for example,olivine, which is a relatively expensive magnesium silicate within therange of forsterite to fayalite. Although such sand substitutes improvethe qualities of low carbon, low alloy steel castings, they areexpensive and have not completely solved the difficulty.

Another prior art approach to this problem has been the incorporation ofceramic powder in the shell mold mixture. After formation of the shellmold, the phenolic resin has been burned out either by heating theentire mold or by burning the casting surfaces of the mold with a torch.Although such expediencies have eliminated the characteristic surfacedefects of low carbon, low alloy steel castings produced in shell molds,the expense of such processes is prohibitive, and it has been found thatthey tend to destroy the dimensional accuracy of the molds.

According to the present invention, it has been discovered that thecharacteristic surface defects of such low carbon, low alloy steelcastings results from a reaction between the molten steel and the mold,and between components of the mold. It is believed that this reaction isat least partially caused by the gases produced in the formation ofsilicon carbide by combination of carbon in the phenolic resin with thesand, and it is further believed that the molten steel participates inthis reaction by developing high temperatures sufficient to cause theproduction of such gas, and possibly the carbon in the steel accentuatesthe formation of such gas by participating in the reaction. It has alsobeen discovered that the molten steel tends to pick up carbon from themold when this reaction occurs, thereby resulting in undesirable carbonfluctuation in portions of the surface of the steel castings.

The use of mold washes to prevent the steel from contacting the shellmold has generally been regarded as impractical due to the fact thatsuch washes destroy the dimensional accuracy of the mold which isproduced to very close tolerances.

Accordingly, the tion is to eliminate the characteristic surface defectsof low carbon, low alloy steel castings formed in conventional shellmolds, without the necessity of altering the conventional shell moldmixture and Without the necessity of burning the phenolic resin from themold after formation thereof.

A more Specific object of the invention is to wash the castingcontacting surfaces of a conventional shell mold Without destroying thedimensional accuracy thereof.

Another object of the invention is to wash the casting contactingsurfaces of a shell mold with a material which penetrates the surfacesand is preferably inert with respect to the molten steel and withrespect to the phenolic resin and the sand in the presence of moltensteel, to prevent the formation of gases or other phenomena causing thecharacteristic surface defect of low carbon, low alloy steel castingsproduced in conventional shell molds.

A further object of the invention is to devise a mold wash in the formof a slurry of water, and a wetting agent, such as, for example, alcoholand finely divided material such as titanium dioxide, manganese dioxideand cerium oxide in a particle size capable of penetrating the castingsurface of a shell mold to a sutficient depth to prevent the reactionwhich causes the characteristic surface defects of low carbon, low alloysteel castings.

it has been discovered, in this connection, that grades of cerium oxide,manganese dioxide and titanium dioxide powders are ineffective for thepurpose above described when the particles range in size from about 7 to15 microns or greater. However, it has been found that manganesedioxide, titanium dioxide and cerium oxide powders, having a. particlesize of from about .2 to about .75 micron, substantially eliminate suchcharacteristic defects of low carbon, low alloy steel castings producedin shell molds, the casting contacting surfaces of which have beensealed with a slurry of such powder and a wetting agent in water. Inthis connection, it has been found that the maximum efiiective particlesize of such powders when used on conventional shell molds is about onemicron, although it will be understood that the size of the pores in theshell mold determines the effective size of the wash particles whichmust penetrate the pores to be effective for the purposes above setforth. The penetration is necessary to provide a barrier to reaction ofthe shell components beneath the casting contacting surfaces thereofwithout developing a layer on the casting contacting surfacessubstantially thicker than 20 microns.

It has been found that the wash particles are most effective when thecasting surface of the shell mold has been penetrated by such particlesto a depth of at least of an inch and preferably of an inch.

According to the invention, after a conventional shell mold has beenproduced in the usual manner, as above described, the casting contactingsurfaces of the mold which define the casting cavity in which thecasting is to be formed by molten steel poured in the usual manner arewashed, as for example, by brushing, spraying, or flushing with a slurryof particles, such as titanium dioxide, manganese dioxide, or ceriumoxide and. a wetting agent in water. The wash is applied until it haspenetrated the casting surfaces of the mold, but has not built up alayer on such surfaces in excess of 20 microns. The washed shell is thendried by the application of heat as, for example, by heating in an oven,or by infra red drying, or by application of a torch primary object ofthe present invenflame to the layer. It has been discovered that thisprevents the reaction which causes the characteristic surface defects oflow carbon, low alloy steel castings without destroying the dimensionalaccuracy of the mold.

The shell mold is then filled in the usual manner with molten lowcarbon, low alloy steel herein defined as a steel having a carboncontentnot substantially in excess of about 1.5 percent by weight and a contentof chromium, manganese, nickel, molybdenum, copper, vanadium, titaniumand aluminum, singly or in any combination thereof within a range ofabout nil to 15 percent by weight. It has been found that steel castingsproduced in this manner are substantially free from the characteristicsurface defects of low carbon, low alloy steel castings formed in shellmolds according to conventional practices.

A Wash which has been particularly effective for the purposes abovedescribed has been devised in accordance with the following formulation:

150 parts by weight of water 100 parts by weight of titanium dioxide,particle size about..4 micron of a part by weight of cellulose gum 7 /2parts by weight of a phenolic resin core binder.

What is claimed is:

1. A method of producing low carbon, low alloy steel castings comprisingmaking a shell mold having sand particles bonded together by athermo-setting resin, said mold having. a porous surface. defining acontour of a' casting tially in excess of 1 micron and then fillingsaidmold with said molten steel.

2. A method according to claim 1, wherein the layer particles aretitanium dioxide.

3. A method according to claim 1, wherein the layer particles aremanganese dioxide.

4. A method according to claim 1, wherein the layer particles are ceriumoxide.

References Cited in the file of this patent UNITED STATES PATENTS2,169,386 Hall Aug. 15, 1939 2,491,096 Feagin Dec. 13, 1949 2,544,598Kalina Mar. 6, 1951 2,592,337 Robertson et al. Apr. 8, 1952 OTHERREFERENCES The Iron Age, Oct. 30, 1952, pages 8892.

1. A METHOD OF PRODUCING LOW CARBON, LOW ALLOY STEEL CASTINGS COMPRISINGMAKING A SHELL MOLD HAVING SAND PARTICLES BONDED TOGETHER BY ATHERMO-SETTING RESIN, SAID MOLD HAVING A POROUS SURFACE DEFINIGN ACONTROUR OF A CASTING TO BE FORMED IN SAID MOLD, THEN FORMING A LAYER ONSAID SURFACE PENETRATING THE PORES THEREOF TO A DEPTH OF AT LEAST 1/04OF AN INCH AND DEFINING A COATING ON SAID SURFACE NOT SUBSTANTIALLY INEXCESS OF 20 MICRONS, SAID LAYER BEING FORMED OF PARTICLES CONSISTING OFONE OR MORE OF THE GROUP OF TITANIUM DIOXIDE, MANGANESE DIOXIDE ANDCERIUM OXIDE, SAID PARTICLES BEING CHEMICALLY INACTIVE WITH RESPECT TOSAID SURFACE IN THE PRESENCE OF SAID STEEL IN THE MOLTEN STATE ANDHAVING A PARTICLE SIZE NOT SUBSTANTIALLY IN EXCESS OF 1 MICRON AND THENFILLING SAID MOLD WITH SAID MOLTEN STEEL.